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You need to know three factors. The track voltage or voltage available to light the bulb, the voltage rating of the bulb and the current the bulb draws. If the track voltage is higher than bulb voltage then you need a resistor or voltage regulator. To determine the resistor value required, subtract the the voltage rating of the bulb from the track voltage. Divide that number by the current rating or draw of the lamp or device. For example: 14 volts track voltage - 1.5 volt bulb = 12.5 volts. Divide by .02 (20 millamp) = 625 ohms. Resistors can get warm so don't alow it to touch plastic. A 1/4 watt resister is fine for a low draw bulb but since you have plenty of room, use a 1/2 watt resister for better heat disaipation.
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